President Barack Obama sought to reassure and inspire Bostonians reeling from the deadly marathon bombing, telling them that America stood with their grieving city and promised: "We'll keep going. We will finish the race."

With Holy Cross Cathedral packed with first responders, families of victims, political luminaries past and present, and members of the public, Obama called Thursday's interfaith service a chance to "mourn and measure our loss."

Standing at the podium, he said the bombing, being investigated as a terrorist act, was personal for millions of Americans who, in a myriad of ways, identify with the Hub.

"Every one of us stands with you," he said. "Boston may be your hometown -- but we claim it, too."

Injecting the experiences of he and his wife, Michelle, as law students just across the Charles River at Harvard and as a rising political figure at the 2004 Democratic convention, Obama said Boston's spirit remains "undaunted and the spirit of this country shall remain undimmed."

The moment is something Obama, like other presidents before him, has become familiar with in the wake of national tragedy.

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Among disasters on Obama's watch: A tornado in Joplin, Missouri, and mass shootings at Fort Hood, Texas; Tucson, Arizona; Aurora, Colorado, and last December's school massacre in Newtown, Connecticut.

Consoler-in-chief is a role modern presidents have become accustomed to filling, said Julian Zelizer, a Princeton University historian and CNN contributor.

"It's an important role for presidents," Zelizer said. "Part of what the country wants is a figure who gets us through a particular crisis."

Obama's appearance at Boston's main Catholic church and visits with some of those injured at hospitals was an inflection point between the tragic chaos of three days ago and the intensifying investigation, which entered a new phase on Thursday with the release of a video by authorities showing two men they call suspects.

"We come together to pray and mourn and measure our loss," Obama said. "But we also come together today to reclaim that state of grace -- to reaffirm that the spirit of this city is undaunted and the spirit of this country shall remain undimmed."

The service was attended by Mitt Romney, Obama's Republican rival in last year's election and a former Massachusetts governor and a Boston-based business executive.

"I thought the president gave a superb address to the people of this city and the state and the nation," Romney said. "It was an inspiring day."

Obama's tone on Thursday was markedly different from the day before when he invoked the memory of mass shooting victims following a setback in the Senate for gun control priorities he supported.

He called that legislative outcome "a pretty shameful day in Washington."

Obama has been frustrated many times on the legislative front since taking office in 2009, but it has never hampered his ability to communicate to the public that he is in control, Zelizer said.

"There's a little bit of a preacher in him and that's something that has always drawn some people to him," Zelizer said. "It gets more powerful and poignant as the speech goes forward."

And as he has on previous occasions when tasked with rallying mourners, Obama vowed that the nation's spirit remains undaunted.

"Of that I have no doubt, you will run again," Obama said to applause in Boston as he projected optimism. "You will run again, because that's what the people of Boston are made of."